My truck has a very slight vacuum leak. The vents switch to defrost, but only when the conditions are extreme. In moderate weather, it never does it. Only when its very cold outside (below -10) and only when lugging up a hill or driving into a headwind before the auto tranny shifts down. Cruise control acts up a little in these conditions too, but still works for the most part. If the cold weather and engine lugging don't occur, everything works fine.

Question 1: Can I find this leak myself?
Question 2: If I don't fix this leak, am I allowing dirty, unfiltered air into the engine?

 

Sounds like you have either a vacuum leak at the vacuum reservoir or a cracked reservoir. It's located under the wiper cowl on the passenger side. Before pulling the wiper cowl, check all vacuum hoses going to and into the firewall for leaks or cracks.

It stores vacuum when the engine is under a load or at WOT because at those times the engine is not creating any vacuum. The vents switching to the default settings of defrost and the cruise control acting up are both related to a lack of vacuum in those situations.

 

1 check valve (the one with the 45 degree outlet), has a very small amount of leakage, and the PCV valve didn't seem to be doing much of anything. I can blow through it fine both ways. Dealer had to order the check valve, so both those will get put on tomorrow. Should I still remove the cowl to get a look at this resevoir, or just hold off till after tomorrow?

 

Repair the known problems first and see if the situation changes. If things remain the same then look into the reservoir as maybe being the problem.

 

if you do remove the cowl, remove windshield wipers by folding the wipers ALL the way upwards, then sliding the clip at the base a small amount, then lift the wiper off the splined shaft.

before removing, mark the position so you get it back in the right spot on the first try. if you don't lift the wiper arm vertical, the clip won't slide. it took me about 15 minutes to figure this out.

about the only thing that can wrong with the vac reservoir is a rotted out vac hose.

the check valve and vac hose in the hot engine compartment is a much more likely culprit.